Waterway scour is the scouring or evacuation of a waterway bed at or near waterway structures, typically caused by the flow of water along the waterway bed. When scour occurs at or near structures associated with bridges, it is referred to as bridge scour. (See "Scoping Out Scour," Civil Engineering Magazine, March 1993.)
Waterway scour is generally segregated into three types: general, constriction and local. General scour includes standard erosion due to general changes in water flow and sediment supply to a broad area. General scour does not usually include scour caused by bridges themselves or other man-made structures. Constriction scour is scour caused by increased water velocity due to the constriction of water flow. Local scour is scour around the bases of pilings and piers caused by turbulence from the pilings and piers themselves.
Scour is known to have serious detrimental effects, particularly near bridges. As the waterway bed is washed away near bridges, stability of the bridges can be reduced to the point of collapse. Measurement of scour depth is therefore useful in monitoring stability and repair needs for bridges and other waterway structures.
A determination of scour depth typically requires more than a simple water depth measurement. For example, after scour occurs near a piling or pier, sediment or infill may accumulate in the depression forming a secondary surface above the scoured waterway bed. While appearing visually to provide a continuous, relatively smooth waterway bed, the infill is often relatively soft and unstable. The infill provides only minimal support for pilings supposedly supported by the waterway bed. Thus, measurements which are unable to detect the actual scour depth of the waterway bed, as opposed to the depth of the secondary surface presented by the infill are of limited value.
Several techniques have been proposed for measurement of bridge scour, including subsurface interface radar, transducers, optical fathometers, physical probes, and visual inspection. All of these systems suffer from significant drawbacks.
Subsurface interface radar systems use transmitted electromagnetic pulses to perform scour depth measurements. In these systems, the electromagnetic pulses are directed through the water into the waterway bed. At the surface of the waterway bed and/or at interfaces between various subsurface layers, such as between infill and the waterway bed or between layers of differing materials in the waterway bed, reflections occur. The reflections are detected by the radar system and analyzed to produce information about the subsurface. In addition to being complex and expensive, the effectiveness of such systems are affected by the materials in and around the waterway bed. For example, subsurface interface radar systems provide limited accuracy in dense, moist clays and are ineffective in salt water. Such systems are also ineffective in other situations where the subsurface is conductive.
Acoustic systems utilize a transducer to transmit acoustic waves through the water into the subsurface. Reflections from the waterway bed and from within the subsurface are detected at the surface and provide information about the scour depth. Such systems require a power supply and relatively complex electronic devices. This makes them relatively expensive to produce and operate. Another difficulty inherent in such systems is that high concentrations of sediment in suspension scatter and absorb the acoustic pulses, making reflections difficult to detect or quantify.
Optical inspection of scour depth, including using divers or a submarine camera, provides some information about scour depth. As described above, such visual techniques provide no information about the subsurface and the presence of infill. This approach also requires expensive specialized equipment and expensive skilled labor. They also pose danger to divers in locations with high currents or traveling debris.
Still another method developed at the University of New Zealand as described in Breusers and Raudkivi (Breusers and RaudKivi, Scouring, A. A. Balkema, Rotterdam, The Netherlands, 1991.) employs radioactive sources and detectors. In this approach, a guide is driven into the waterway near a bridge piling. A weight containing a radioactive source is slidably attached to the guide and allowed to descend to the waterway bed. The radioactive weight rests upon the bed of the initial scour hole and slides down the outside of the guide as the scour hole deepens. As sediment fills the scour hole, the lead weight is covered by the sediment. To determine the depth of the radioactive weight, Raudkivi inserts a gamma ray probe inside of the guide and slides the probe down while trying to detect gamma ray radiation from the radioactive source. By monitoring the depth of the gamma ray probe, Raudkivi obtains a measurement of the scour depth.
The radioactive approach of Raudkivi provides a relatively accurate measurement of scour depth, while requiring a minimum of skilled labor. However, the system utilizes expensive, complex electronics and requires skilled labor to operate. Further, the use of radioactive materials poses a wide range of environmental and regulatory problems.